This disclosure relates to thin film photovoltaic solar cells. Photovoltaic cells or solar cells are photovoltaic components for direct generation of electrical current from sunlight. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years and continues to expand. Solar cells include a substrate, a back contact layer on the substrate, an absorber layer on the back contact layer, a buffer layer on the absorber layer, and a front contact layer above the buffer layer. The layers can be applied onto the substrate during various deposition processes.
The buffer layer can serve two functions that include (1) providing an n-type semi-conductive layer and (2) reducing a shunt current. As such, the buffer layer can include a metal chalcogenide, such as cadmium sulphide (CdS) and zinc sulphide (ZnS), in chalcopyrite (CIGS) and kesterite (CZTS) solar cells. The metal chalcogenide thin films can be prepared by a chemical bath deposition (CBD) process in a deposition tank or chamber. The CBD process includes using a reaction solution that includes thiourea and ammonia mixed with a metal sulfate, such as cadmium sulfate (CdSO4) or zinc sulfate (ZnSO4), a metal nitrile, or a metal chloride in distilled water at a temperature range of from about 50 to about 90 degrees Celsius.
During the CBD process, the concentration values of the various chemical components of the reaction solution can change and some residual compounds can form. For example, sodium or other elements can slightly dissolve from the chalcopyrite absorber during the CBD process. These elements will accumulate and, as a result, the concentration values of various chemical components in the reaction solution can increase. Accordingly, after a CBD process has completed for one substructure, the chemicals are drained out from the deposition chamber and new chemicals are added prior to the commencement of another CBD process for a different substructure.